Silicon carbide has recently attracted attention as a wide band gap emitter material for silicon heterojunction bipolar transistors. Si-HBTs. SiC emitters have been shown to block hole back injection in npn transistors allowing heavily doped base regions to be used, while maintaining reasonable current gain. This in turn permits a narrowing of the base region, improving high frequency performance.
Conventional known methods for formation of device quality single crystalline .beta.-SiC require epitaxial growth by reaction of silanes and a hydrocarbon, typically acetylene or propane, at temperatures higher than 900.degree. C., and typically 1400.degree. C., for example as described in U.S. Pat. No. 4,923,716 to Brown et al. issued 8 May 1990, entitled "Chemical Vapour deposition of silicon carbide". Such high temperatures are undesirable in a bipolar CMOS (BiCMOS) process for forming wide band gap emitters in Si-HBTs, and in particular for high speed bipolar transistors which have a very thin (about 50 nm) highly doped base. To suppress dopant redistribution in the base and preserve ultra shallow base profiles during emitter formation, alternative low temperature processes for deposition of amorphous and polycrystalline SiC.sub.x deposition are required.
Polycrystalline silicon carbide emitters for HBTs have been formed at a deposition temperature as low as 900.degree. C. as described by T. Sugii, T. Aoyama, Y. Furumura, and T. Ito, Proceedings of the First Topical Symposium on Silicon Based Heterostructures, edited by S. S. Iyer et al., Toronto, Canada, October 1990, pp. 124. Also, as described in the latter article and references therein, amorphous SiC emitters were fabricated by deposition at 700.degree. C., but the material was heavily doped with fluorine to passivate dangling bonds. In the latter process, in which SiC.sub.x was deposited from a gaseous reactant mixture of disilane Si.sub.2 H.sub.6 and acetylene C.sub.2 H.sub.2 with phosphine PH.sub.3, and difluorosilane SiH.sub.2 F.sub.2 as a source of fluorine, acceptably low film resistivity could only be obtained if the atomic carbon fraction were 20% or less.
As described in U.S. Pat. No. 5,053,255 to Boeglin entitled "CVD process for the thermally depositing silicon carbide films onto a substrate " issued 1 Oct. 1991, silicon carbide may be deposited at low temperature by pyrolysis of di-tert-butylsilane (DTBS). DTBS is a less toxic, air stable, non corrosive liquid, and is thus preferred over silane and other gaseous sources of silicon for CVD (chemical vapour deposition) being less hazardous in use. However, the latter process was found to produce silicon carbide films with an appreciable oxygen content .about.6%. An alternative method using Plasma CVD as described in U.S. Pat. No 5,061,514 to Boeglin entitled "CVD process for the plasma depositing silicon carbide films onto a substrate", issued 29 Oct. 1991 allows reaction to be carried out at a lower temperature, in the range from 100.degree. C. to 400.degree. C., but the resulting film was carbon rich and contained a substantial amount (12%) of oxygen. In forming Si-HBTs the presence of oxygen may create generation-recombination centres which may increase junction leakage.
Thus for application in Si-HBTs, a process with a reduced thermal budget (i.e. process temperature and time product) is required to form emitter quality SiC.sub.x with low resistivity, i.e., by incorporation of controlled amounts of impurities comprising, for example, phosphorus or boron, and fluorine, and reduction of other impurities which may be detrimental to electrical characteristics, e.g., oxygen.